The invention relates to a device for bonding two or more metal plates, strips, and the like by absorption of energy from laser radiation, provided by beam-forming optical means.
Devices for bonding are known of themselves. In bonding operations, high-quality coating materials that possess particular chemical, physical, or mechanical properties are bonded with inexpensive base materials that contribute strength. It is necessary to achieve a strong metallic connection between the coating material and the base material. In the past this has been accomplished by squeezing the coating material onto the base material in a plastic state. The energy required for this purpose is applied for example by rolling or explosions in the bonding zone.
Bonding is used to produce semifinished products composed for example of two or more metal layers bonded together by the methods of cold-roll bonding, hot-roll bonding, explosive bonding, or a combination of these bonding methods. The semifinished products are therefore called bonded materials and belong to the class of layer bonded materials. The metal layers of the coating material and base material are usually much thicker than those employed in conventional surface treatment methods. In bonded layered materials the technical and economical properties of their components are combined in such fashion that suitability for the purpose is obtained that is superior to that of the individual materials.
In hot-roll bonding or cold-roll bonding, average to high rolling pressures on the order of 10.sup.2 N/mm.sup.2 or 10.sup.4 N/mm.sup.2 are used. The sheet widths that can be bonded in this fashion are less than 0.6 m. In explosive bonding, an explosive pressure of 10.sup.5 -10.sup.6 N/mm.sup.2 is used. Important disadvantages of explosive bonding however include the wavy structure of the bonding zone between the coating material and the base material and a sheet width of 3 meters or more.
One disadvantage of cold rolling bonding and hot rolling bonding is the fact that only metal plates with widths up to 60 cm can be processed. In addition, in hot-roll bonding and cold-roll bonding, the high roller pressures already mentioned must be used, resulting in a corresponding cost of machinery and energy. In the hot-roll bonding method, the coating material and base material are heated to 1200.degree. C. so that a change takes place in the metallurgical and mechanical properties of the materials.
A device and a method for bonding using laser radiation is known from German Patent 37 13 975. The device disclosed therein and the corresponding method are proposed in particular for welding the edges of workpieces, but are also intended to perform bonding of layered materials. In German Patent 37 13 975 the device uses laser radiation that is particularly suited for heating the surfaces of two layered materials fitted together at a V-shaped gap. This laser radiation runs essentially parallel to the plane of the gap of the joint and perpendicular to the line of the joint. In addition, the laser radiation oscillates primarily parallel to the plane of the gap of the joint. According to the subject of German Patent 37 13 975, the surfaces of the two materials to be joined are brought closer and closer together to produce the joint. A welded seam extends from this joint, with the surfaces of the materials to be joined being welded together in the vicinity of the welded seam by the melting of the materials. The surfaces of the materials are brought together by applying pressure in the vicinity of the joint. In the area of the joint itself, the surfaces of the materials abut one another with practically zero pressure so that the molten material of the two material surfaces is under practically no joining pressure. The method for joining two metal layers according to German Patent 37 13 975 makes sense for welding but not for bonding involving two metal layers. In bonding, the surfaces of the two metal layers to be joined are not heated to the melting point, and the temperature is deliberately kept below the melting point of the workpieces to be joined since bonding takes place only when the materials to be joined are in the plastic and not the molten state. In addition, during the bonding process the heated zones of the surfaces of the two metal layers to be joined are pressed directly against one another by rollers in order to produce a bond between the two surfaces. It is not possible in bonding to allow the heated zones of the two metal layers to be joined to rest against one another at the joint with zero pressure, as is the case in the subject of German Patent 37 13 975.
In the subject of German Patent 37 13 975, a CO.sub.2 laser is used as the source of laser radiation. In a CO.sub.2 laser, the power requirement for a bonding width of about 1 cm is currently between 1-4 kW depending on the process and the materials used. For a bonding width of more than 1 meter, laser powers greater than 100 kW are required, which cannot be produced using the laser systems that are currently available. In addition, bonding of metal layers must be performed without local melting of the surfaces of the two material surfaces to be joined. This imposes extreme demands on the homogeneity of the power density distribution at the joint. In CO.sub.2 lasers, homogenization is produced by beam-forming optics and/or vibrating mirrors. Homogenization will be very difficult for example by using faceted mirrors for technical reasons (Fresnel number) for the wavelength of the CO.sub.2 laser. If metal plates more than 1 meter wide are to be joined by bonding, a considerable technical and equipment cost is also required that entails correspondingly high costs to achieve homogenization of the power density distribution at the joining point of the two surfaces.
U.S. Pat. No. 5,306,890 teaches a device for welding a smooth metal sheet and a corrugated metal sheet. These two sheets are welded together, with the smooth sheet serving as the support for the corrugated structure of the other sheet. The welded product thus produced is used in catalytic converters fitted to automobiles with internal combustion engines. The width of the support with the corrugated sheet must not be less than 160 mm. A YAG laser or a CO.sub.2 laser is used for welding. Beam-forming means are provided for the laser beam, so that the metal sheets to be bonded are irradiated along the joint line over their entire lengths. The two metal sheets are heated in the vicinity of the joint line and squeezed together by two rollers at the joint line. The thickness of the two metal sheets is between 50 microns and 1 millimeter. The laser beam is extended over the entire joint line by using a mirror suitably designed according to the prior art. U.S. Pat. No. 5,306,890 is a welding method to which the subject of the present application is not directed.
The subject of DE-OS 43 01 689 is a semiconductor laser system that comprises a plurality of semiconductor laser units, each of which has a coupling element that couples the laser radiation of the respective semiconductor laser unit into a light-conducting fiber and a fiber bundle comprising the fibers as a light guide system that has one end from which a total laser radiation composed of the combined laser radiation created by the semiconductor laser units emerges, said total laser radiation illuminating a target area on an object to be irradiated when all the semiconductor laser units are engaged in laser activity. A control is provided with which the power of each individual semiconductor laser unit can be controlled in a specific fashion, and for control purposes the irradiation of different surface elements of the target surface can be defined with an intensity that can be set individually for each surface element. For control, an irradiation profile that varies in time and/or space can be preset for the target area. The subject of DE-OS 43 01 689 can be used to perform irradiation tasks such as irradiating different surface elements within the target surface at different intensities and thus adapting the radiation on surface areas of the target surface to the desired brightness. The total laser radiation is not produced by one laser system but by a plurality of semiconductor laser systems, thus permitting complex irradiation by specific control of the power of the individual semiconductor laser units.
In one embodiment of the semiconductor laser system, in addition to the fiber bundles, additional detector fibers are provided for illumination that are located in the end area, with their fiber end surfaces arranged in a regular fashion between the fiber end surfaces for illumination, so that the additional detector fibers produce an image of the fiber end surfaces on the target surface takes place in the same manner as in the fiber end surfaces for illumination. The ends of the additional detector fibers opposite the fiber end surfaces terminate in a detector matrix that detects the received radiation individually for each additional detector fiber. With this detector matrix, therefore, an image of the target area can be obtained and an image of the target surface can be displayed on a screen by means of a corresponding image processing unit. For this purpose, a number of additional detector fibers are worked into the fiber bundle for illumination such that a sufficiently accurate representation of an image of the irradiated target surface on the screen is possible and thus allows exact observation of the irradiated surface areas of the target surface. Measures for homogenizing the power density distribution of the laser radiation are not provided in the subject of DE-OS 43 01 689.
A method is known from JP 2-247096 A for rendering uniform the intensity of laser radiation, and is not described in greater detail. For this purpose, the laser beam is conducted via a mirror into a device that displaces the optical axis. This radiation is conducted through a lens into a calite scope where it is reflected several times. This multiple reflection results in the power density distribution of the laser beam being rendered uniform. A welding method for stainless pipe is known from JP 4-258390 A. A laser beam not defined in greater detail is used for welding. Before the two edges of the sheet are welded together to form a pipe, these ends or the entire pipe are/is preheated using an electrical system that uses high frequency. This produces a uniform weld.
Hence, the goal of the invention is to provide a device for bonding at least two metal plates, strips, or the like, which in particular can bond plate widths of the coating material and base material that can be even wider than 1 meter, said device also meeting the highest requirements for homogeneity of the power density distribution at the joint, said device permitting minimization of the changes in the metallurgical and mechanical properties of the metal plates to be bonded, which prevents melting during bonding and failure of the two surfaces to be joined, which permits a reduction of the roller pressure on the two metal plates to be joined, and finally allows a reduction of the energy required for bonding the two metal plates to be joined.
The advantages of the invention consist in particular in that by using a radiation source that is formed by an arrangement of laser diodes, as a result of the far higher electrical and optical efficiency of the laser diodes, an energy saving of five to six times is possible by comparison with bonding using a CO.sub.2 laser with approximately the same degree of absorption. Another very important advantage of the invention consists in that, in order to homogenize the power distribution of the laser beam at the joint, along the entire length of the joint line according to the invention glass plates with total reflection for the laser beam passing through or prisms joined wedgewise, likewise with total reflection in the prism for the laser beam passing through, or a very dense arrangement of laser diodes extending lengthwise and stacked above one another in several planes with laterally applied reflecting limiting devices or special light guide fiber bundles are used. By placing the glass plates, prisms, or reflecting limiting surfaces or special light guide fiber bundles between the laser beam source and the cylindrical lens that focuses to a rectangular beam cross section, a very high homogeneity of the power density distribution can be achieved over the entire length of the rectangular focus or joint line that cannot be achieved with conventional bonding methods or even with a CO.sub.2 laser beam that moves back and forth over a width of more than 1 meter. By virtue of the special arrangement of the laser diodes in laser diode bars that can be lined up side by side as desired, and which also can be stacked vertically, with the aid of the cylindrical lens a laser radiation can be achieved with a rectangular beam cross section that heats only locally limited zones of limited spatial depth at the two surfaces of the metal plates to be bonded in the V-shaped gap between the surfaces of the metal plates to be bonded. As a result of the limited spatial depth of the heating at the joint, it is possible to achieve a very low change in metallurgical and mechanical properties of the two metal plates to be bonded, in other words the surface material and the base material. In addition, the uniform power density distribution of the laser radiation with a rectangular beam cross section as well as measurement of the laser end or heat radiation provided at the joint makes it possible to generate a regulating signal to change the feed rate of the metal plates and/or the laser power, to avoid melting of the surfaces of the two metal plates to be joined, and always to maintain a plastic state of the materials that allows optimum joining of these two surfaces. As a result of this exact control of the plastic state of the surfaces of the two metal plates to be joined, rolling pressures that are low by comparison with hot- and cold-roll bonding are required to press together the two metal plates to be joined. As a result, only low degrees of deformation take place, resulting in a reduction of the solidification in the bonding zone between the two metal plates to be joined. By virtue of the special arrangement of the laser diodes arranged side by side in laser diode bars, widths of more than one meter for the metal plates to be joined can be achieved at any time without additional technical measures. The high laser power required for a corresponding application can be varied as desired by stacking these laser diode bars made of rows of diodes, one on top of the other.
The invention will now be described with reference to embodiments and drawings.